Due to a growing environmental awareness worldwide, the concept of product stewardship is rapidly becoming a reality for many manufacturing companies. It is no longer considered acceptable to be concerned only with the environmental consequences of a particular manufacturing process or the hazards associated with a particular chemical. Increasingly, industry is recognizing that public opinion dictates that they be held accountable for the environmental fate of their products after their intended function is complete. At the moment the need to be responsible product stewards is primarily driven by a response to public perception, but it is not at all unreasonable to envision that public opinion will soon be replaced by legal mandates.
The technical difficulties associated with being a responsible product steward are extremely complex. The chemical industry is being asked to both provide products with the performance characteristics that the public has grown to expect, and also products which will not persist in the environment. Viewed from current polymer technology, these product requirements are often mutually exclusive. Thus far, the response of individual chemical companies has been largely configured around their existing product streams. For producers of commodity goods, such as plastics used in packaging, the focus has been mainly on recycling. However, it is becoming increasingly apparent that for many disposable items, recycling or incineration do not represent feasible options. Cigarette filters are a classic example of a product type for which it would be extremely difficult to design and implement an effective collection and recycling or disposal program. Discarded cigarette filters represent a significant surface litter problem, even in areas where proper disposal receptacles are conveniently available. Thus, there is a critical market need for biodegradable materials that will not persist in the environment.
The term "biodegradable" is becoming an increasingly popular label for manufacturers to place on their products. Unfortunately, its application in many cases is inaccurate and misleading. As a direct result of the unregulated use of this term, environmental groups and the public have generally come to distrust a manufacturer's claims regarding biodegradable commodities. This situation is further augmented by the total lack of standards or legal mandates dealing with biodegradable polymers (Donnelly, J. 1990. Garbage, June:42-47). For the purpose of this invention, a precise definition of "biodegradable polymer" is provided in order to prevent any possible misinterpretations.
First and foremost, biodegradation is a biologically mediated process; it thus requires the direct interaction of microorganisms and/or their enzymes with the polymeric substrate. Without a biological component, use of the term "biodegradable" is a misnomer. Polymer biodegradation typically begins with a series of microbial catalyzed chain cleavage steps producing lower molecular weight fragments. These fragments are then further metabolized to short chains or monomers, which can be assimilated by the microbes and used as sources of carbon and energy. Obviously, as the degradation process continues, significant physical changes in the native polymer become apparent. Traditionally changes in physical characteristics, such as tensile strength, have been used as the sole criteria for evaluating the inherent biodegradability of a polymer. However, the most stringent requisite for determining biodegradability is total mineralization of the polymeric carbon to CO.sub.2 and H.sub.2 O. In other words, a quantitative transfer of carbon from the polymeric chain to microbial biomass and/or their metabolic end products has to take place--with no persistent (non-biodegradable) residues. Under aerobic conditions, the metabolic path to mineralization is usually direct. In contrast, anaerobic metabolic systems typically produce metabolic end products such as methane and volatile fatty acids. These components are also non-persistent and will eventually be converted into CO.sub.2 by means of less direct microbial systems.
The term "biodegradable polymer" as defined above automatically eliminates many products that merely undergo particle size reduction but yield persistent residues. For example, starch polyethylene blends have been commercially sold as biodegradable products. In actuality, only the non-sequestered starch is biodegradable. Even though microbial metabolism of the available starch is responsible for significant particle size reduction, the ultimate fate of these particles has to be taken into consideration. Both the polyethylene and the sequestered starch are recalcitrant to microbial enzymes, which means they will persist in the environment, negating the manufacturer's claim of biodegradable (Donnelly, J. 1990. Garbage, June:42-47).
In addition to the degradation potential of the polymeric substrate, other important chemical and physical requirements of the microorganisms must be met in order for successful biodegradation to occur (Glenn, J. 1989. Biocycle, October:28-32). Microorganisms represent an extremely diverse group, having adapted to a vast array of environmental extremes. However, all cells have obligate requirements before they are able to survive and grow. Examples include suitable pH, temperature, ionic strength, the proper oxygen concentrations (or the lack of oxygen for anaerobic species), available macro and trace nutrients, and appropriate moisture levels. The exact requirements will obviously vary with different species. It is important to highlight that the term biodegradable is not a universal constant that applies equally to all situations and under all environmental conditions. One only has to consider the poor performance of highly biodegradable materials in a typical landfill setting to fully appreciate this point (Donnelly, J. 1990. Garbage, June:42-47). This fact is often overlooked when examining the poor performance of inherently biodegradable materials in sub-optimal environments. If a compound biodegrades when placed in a suitable environment, this potential does not disappear when it is placed in a different environment, only the rate at which it degrades will change.
Numerous studies have demonstrated that cellulose or cellulose derivatives with a low degree of substitution (DS), i.e., less than one, are biodegradable. Cellulose is degraded in the environment by both anaerobic and aerobic microorganisms. Typical end products of this microbial degradation include cell biomass, methane (anaerobic only), carbon dioxide, water, and other fermentation products. The ultimate end products will depend upon the type of environment as well as the type of microbial population that is present. However, it has been reported that cellulose esters with a DS greater than about one are completely resistant to attack by microorganisms. For example, Stutzenberger and Kahler (J. Appl. Bacteriology, 66, 225 (1986)) have reported that cellulose acetate is extremely recalcitrant to attack by Thermomonospora curvata.
It is well-known in the art that cellulose esters such as cellulose acetate (CA) are widely used in applications such as cigarette filters. The CA fibers used in cigarette filters and other applications typically contain finely ground pigments at concentrations ranging from 0.5-2.0% (wt/wt). These pigments are added to CA fibers to provide opacity, thus acting as a delusterant or whitening agent. An example of such pigments is titanium dioxide. Generally, two crystalline forms of TiO.sub.2, Rutile and Anatase, are used in the production of CA fibers and the choice depends upon the specific properties desired. In addition to their difference in their crystalline forms, Rutile and Anatase also differ in their specific gravity, refractive index, and hardness as well. Rutile is inherently harder and more abrasive than Anatase because of its higher degree of crystallinity. Hardness is of particular concern because of abrasion which decreases the lifetime of the equipment used to manufacture the fiber. In order to decrease the abrasion of the TiO.sub.2, other materials such as SiO.sub.2, Al.sub.2 O.sub.3, and Sb.sub.2 O.sub.3 are generally used to coat the titanium dioxide. These coatings also improve dispersion of TiO.sub.2 in CA polymers. Furthermore, coating the surface of the TiO.sub.2 decreases the photoreactivity of the TiO.sub.2 thereby lowering the susceptibility of fibers to ultraviolet light which significantly lowers the amount of photodegradation of the fibers on exposure to sunlight (Braun, J. H. J. Coating Technology 1990, 62, 37.).
The steps involved in the manufacturing of cigarette filters is well known to those skilled in the art and is described, for example, by R. T. Crawford, et al. in U.S. Pat. No. 2,794,239 (1957), incorporated herein by reference. Typically, cigarette filters are elongated rods, substantially the size of a cigarette in diameter and circumference, composed primarily of crimped fibers, e.g., cellulose acetate, which are oriented in such a manner that substantially no channels are present which will permit the passage of unfiltered tobacco smoke. The fiber bundle is typically contained within a paper shell or wrapper where the paper is lapped over itself and is held together by a heat sealable adhesive; the adhesive is typically water insoluble. While it is not necessary to use plasticized fiber in forming the filter rods, in practice 2 to 15% plasticizer, e.g., dibutyl phthalate, tripropionin, triethylene glycol diacetate, triacetin, or a mixture thereof are typically applied by either spraying to the surface of the fiber by centrifugal force from a rotating drum apparatus, or by an immersion bath in order to bond the fibers together and to impart additional firmness to the rod. It should be recognized that these plasticizers are water insoluble. Thus, when the used cigarette filter is discarded as surface litter, the fibers of the filter do not disperse which inhibits photochemical or biological degradation.
There exists in the marketplace the need for fibers which would not persist in the environment and which could be used in applications for disposable items such as cigarette filters, agricultural canvas mulch, bandages, infant diapers, sanitary napkins, fishing line, fishing nets, and the like. Moreover, there exists in the marketplace the need for a cigarette filter that, when exposed to a substantial amount of water, will disperse in the environment due to the water solubility of the adhesive, plasticizer, or bonding agent binding the paper wrapper and fibers, respectively, together.